Is Alan the Sentinal over there then, and most powerful being on earth 2 ?
And is OA same as Guardians here on Earth Prime?

There is no Oa that we know of in the Earth-2 universe, but then again there is a Mars and Tamaran out there that are very similar to the ones we see on Earth-1, so I may be wrong! We do know that despite its location Earth-2 as we see it now is actually Telos' world as seen in Convergence, sent into the Earth-2 universe to be home to the Apokolips War Survivors. Alan Scott and his Power Ring have nothing at all to do with Green Lantern rings as we know then, so continuity slip that will never be followed up on or a hint at something to come we shall just have to wait and see.

As with so many dangling loose ends and abandoned plots in this series I am not holding my breath.